The invention relates to carburetors for internal combustion engines and more particularly to automatic cold-start and warmup systems therefor.
Automatic cold-start systems are known wherein a temperature-sensitive element in the form of a bimetal coil experiences the temperature of a positive temperature coefficient (PTC) resistor and is coupled with the carburetor choke valve (U.S. Pat. No. 3,752,133).
For fully satisfactory operation of a cold-start system, the opening of the choke valve should occur in successive steps; first a fast and partial opening of the choke valve immediately subsequent to engine starting so as to lean rapidly the very rich mixture necessary for cold cranking, then a much slower opening during a second phase to keep the mixture at the richness necessary for satisfactory operation of the engine during warmup, and finally a fast opening to maximum opening to cut out enrichment once the engine has warmed up close to its normal temperature.
This object cannot be achieved by providing several PTC resistors in parallel relation, one of which is energized at the starting of the engine and at least another of which is energized by a switch which closes when the engine reaches a predetermined temperature.
It would probably be possible to achieve that result by using a number of resistors to heat the temperature-sensitive element, such resistors being controlled by automatic selection mechanisms, but such mechanisms would be expensive and subject to wear. It is an object of the invention to provide an automatic cold start and warmup system which is improved with respect to the prior art system.